1. Field of the Invention
This invention relates to optical components such as optical fibers for the transmission of mid-infrared ("IR") spectral wavelengths, such as from CO and CO.sub.2 lasers, for high power applications and, more generally, for sensing applications such as sensing near-ambient temperatures, and for molecular spectroscopy. In particular the present invention provides a new approach for creating antireflective surfaces for micro optical components and for the input and output ends of optical fibers used in these applications
2. Information Disclosure Statement
Mid-IR fibers cover the spectral range from 4 .mu.m to 20 .mu.m. [See e.g. V. G. Artjushenko, et al, "Mechanism of Optical Losses in Polycrystalline Fibers", SPIE Proc. 1591, Infrared Fiber Optics III. (1991).] Powerful lasers, such as CO.sub.2 lasers, operate within this portion of the electromagnetic spectrum. Thermal sensing of ambient temperatures takes place there; and the majority of the fundamental vibrational and bending frequencies important to molecular spectroscopy are found there.
While the introduction of silver halide based fibers has improved the transmission, the fibers still suffer from high losses from Fresnel reflections at the fiber/air interfaces. The high refractive indices of the silver halides, typically 2.2, cause reflective losses of 14% per air/fiber interface.
In optics, a common approach to reducing Fresnel losses consists of applying antireflective coatings to an optical component's surfaces by sputtering or chemical vapor deposition. Efforts to use these methods to manufacture end surfaces with antireflective coatings on thin mid-infrared optical fibers, however, have been frustrated by numerous difficulties. Among them are: the limited temperature stability of the fibers, e.g. about 300.degree. C. for the silver halides; the high susceptibility to degradation on exposure to ultraviolet light; the softness of the material; the general lack of adhesion to many coating materials; and mismatches in coefficients of thermal expansion with most of the candidate coating materials. As a result mid-IR optical fibers with antireflective coated ends and generally micro optical components for mid-IR transmission with antireflective coated surfaces do not yet exist.